Fe And Ni Co-doped Gd/SmBaCo₂O_5+δDouble Perovskites As Cathode Materials For Intermediate-temperature Solid Oxide Fuel Cells

Solid oxide fuel cell(SOFC) is an energy conversion device that can cleanly convert chemical energy to electrical energy with high efficiency, which has important applications on solvingserious energy and environmental problems. Traditional operationtemperature of SOFC is ~1000 °C, but in order to decrease the preparation and running cost, and promote the commercial applications of SOFC, it has been concerned issues for science researchers to develop the intermediate-temperature solid oxide fuel cell(IT-SOFC) which operation temperature is 600-800 °C. However, decreasing the operation temperature commonly results in the increased polarization assistance and ohmic assistance for traditional SOFC electrode materials, eventually influences the output performance for overall cell. Hence, it is very important for developing new electrode materials for IT-SOFC, and the most important key is to develop new cathode materials with high performance.A cobalt-free double perovskite GdBaFeNiO5+δ(GBFN) is prepared via sol-gel method and investigatedas a potential cathode for IT-SOFC based on Ce0.8Sm0.2O1.9(SDC) electrolyte. The GBFN sample sintered at 1150 °C for 10 h has a tetragonal structure atroom temperature, and has good chemical compatibility with SDC electrolyte after calcined at 1000 °C for 10 h. XPS analysis indicates that mixed valence states of Fe3+/Fe4+ and Ni2+/Ni3+ coexist in the GBFN material. The absence of spinstate in Ni2+ contributes to a reduction in the thermal expansion coefficient(TEC). The average TEC of the GBFN sample is 14.7 × 10-6 K-1 between 30 and 1000 °C in air, obviously lower the TEC values for GdBaCo2O5+δ(GdBCO) and GdBaCoFeO5+δ(GBCF) materials. The GBFN undergoes a transition from semiconductor to metallically conductive behaviors at 400 °C. The substitution of Fe and Ni for Co significantly improves the structural stability of the GBFN. At 800 °C, the polarization assistance for GBFN cathode and maximum power density for GBFN cathode with NiO-SDC as anode are 0.219 ? cm2 and 287 mW cm-2 on SDC electrolyte, respectively. In addition, the activity and performance of the GBFN cathode are further improved by the impregnation of nano-sized SDC particles: the polarization resistance is decreased by ~14.2 times and the maximum power density of cell is increased by ~1.9 times. At 800 °C, the polarization assistance for 3 impregnations GBFN composite cathode and maximum power density for 3 impregnations GBFN composite cathode with NiO-SDC as anode are 0.065 ? cm2 and 515 mW cm-2 on SDC electrolyte, respectively. After long term stability test at 700 °C for 20 h, the performance of single cells for GBFN cathode and 3 impregnations GBFN composite cathode declines less. Above results suggested that GBFN and its composite material are promising cathodes for applications to IT-SOFC.On the one hand, the Fe and Ni substitutes overall Co greatly decreases the TEC for Co-based perovskite materials, on the other hand, this method also greatly sacrifices the electrochemical performance for cathode materials. Thus, for the sake of balancing the thermal expansion, stability and electrochemical performance of cathode materials, in this paper, Fe and Ni co-doping on Co site double perovskite GdBaCo2/3Fe2/3Ni2/3O5+δ(GBCFN) and SmBaCo2/3Fe2/3Ni2/3O5+δ(SBCFN)oxides are prepared and investigatedas a potential cathode for IT-SOFC based on Ce0.8Sm0.2O1.9(SDC) electrolyte. The GBCFN and SBCFN samples sintered at 1000 °C for 10 h has a tetragonal structure and orthorhombic structure atroom temperature, respectively, both of them have good chemical compatibility with SDC electrolyte after calcined at 1000 °C for 10 h. XPS analysis indicates that mixed valence states of Co3+/Co4+, Fe3+/Fe4+ and Ni2+/Ni3+ coexist in the GBCFN and SBCFN materials. The absence of spinstate in Ni2+ contributes to a reduction in the TECs for G(S)BCFN samples. Compared to the GdBCO and SmBaCo2O5+δ(SmBCO)materials, Fe and Ni co-doping method creates more natural oxygen vacancies. The average TEC of the GBCFN and SBCFN samples are 15.2 × 10-6 K-1 and 16.3 × 10-6 K-1 between 30°C and 900 °C in air, respectively, obviously lower the TEC values for pristine materials and Fe doping cathode materials. The conduct mechanism of GBCFN and SBCFN belongs to small polaron hopping model, the maxmium electrical conductivity achieve 24.7 S cm–1 at 375 °C and 64 S cm–1 at 275 °C, respectively, which exceed the electrical conductivity of Co-free GBFN cathode. Besides, the thermal gravity cycle test indicates the Fe and Ni co-doping on Co site significantly improves the structural stability of the SBCFN sample. At 800 °C, the polarization assistance and maximum power density for GBCFN cathode are 0.187 ? cm2 and 317 mW cm-2 on SDC electrolyte, respectively. At 800 °C, the polarization assistance and maximum power density for SBCFN cathode are 0.128 ? cm2 and 331 mW cm-2 on SDC electrolyte, respectively. In order to further improve the electrochemical performance of SBCFN cathode, the SmBaCo2/3Fe2/3Ni2/3O5+δ–30 wt.%Sm0.2Ce0.8O1.9( SBCFN–30SDC)composite cathode was prepared. The results reveal that the composite cathode significantly decreases the TEC of SBCFN cathode. At 800 °C, the polarization assistance and maximum power density for SBCFN–30SDC composite cathode are 0.046 ? cm2 and 531 mW cm-2 on SDC electrolyte, respectively. Above results suggested that SBCFN and its composite material are promising cathodes for applications to IT-SOFC.